The present invention relates to an image pickup tube, and more particularly to an improvement in the structure of an electrode for collimation lens arranged near a photoconductive target of the image pickup tube.
In conventional image pickup tubes for general use of so-called separate mesh electrode type, a mesh electrode in the most close proximity of a target electrode and a cylindrical accelerating electrode adjacent to the mesh electrode are electrically isolated and applied with different potentials to form a collimation lens. In order to maintain coaxial disposition of the accelerating electrode and the mesh electrode within a bulb of the image pickup tube, in a conventional image pickup tube as disclosed in U.S. Pat. No. 3,073,981 to Louis D. Miller et al. (especially FIG. 2), for example, an insulator is arranged circumferentially of a fore end of the cylindrical accelerating electrode, which end is close to the mesh electrode, and the accelerating electrode and the mesh electrode are jointed together through the insulator. In such a joint, a first support ring is used for mounting the insulator to the accelerating electrode and a second support ring for mounting the insulator to a mesh support electrode extending from the mesh electrode. The first support ring secured to one portion of the insulator is usually fixed to the accelerating electrode by spot welding. Similarly, the second support ring secured to the opposite portion of the insulator is fixed to the mesh support electrode by spot welding.
However, the joint of the accelerating electrode and the mesh electrode in the image pickup tube constructed in the above mentioned manner includes an extremely large number of parts, most of which are joined by spot welding, thus radically complicating the manufacturing process. This not only increases the number of work steps excessively, but also gives rise to a great quantity of wastes and dirt caused in these work steps, particularly by splashes during spot welding. This was naturally undersirable for various properties of an image pickup tube. In the structure as above mentioned, the accuracy of centering for the accelerating electrode and the mesh electrode is subject to dimensional accuracies of the accelerating electrode, the insulator and the support member as they are stacked over each other and therefore, the accuracy of the axial relation between the accelerating electrode and the mesh electrode becomes irregular and at the extremity degraded, so that the image pickup tube properties are deteriorated and particularly the picture distortion properties are aggravated. Further, since the first support ring and the accelerating electrode are fixed together by spot welding as mentioned above, such that the support ring and the accelerating electrode are perfectly jointed to each other at the point where the welding has been performed, they are not jointed at other points, at which gaps equal to the dimensional difference between the outer diameter of the accelerating electrode and the inner diameter of the support ring are created. This results in a polygonal cylindrical shape of the accelerating electrode and disturbance in the electric field in the proximity thereof, thus degrading the image pickup tube properties.